1. Field of the Invention
The present invention relates to initiator systems and IR-sensitive compositions containing initiator systems. In particular, the present invention relates to printing plate precursors which can be developed on-press without requiring a preheat step or a separate development step.
2. Background of the Invention
Radiation-sensitive compositions are routinely used in the preparation of high-performance printing plate precursors. There are primarily two ways of improving the properties of radiation-sensitive compositions and thus also of the corresponding printing plate precursors. The first way addresses improvement of the properties of the radiation-sensitive components in the compositions (frequently negative diazo resins or photoinitiators). The other way deals with improvement of physical properties of the radiation-sensitive layers through the use of novel polymeric compounds (“binders”).
The latest developments in the field of printing plate precursors deal with radiation-sensitive compositions which can be imagewise exposed by means of lasers or laser diodes. This type of exposure does not require films as intermediate information carriers since lasers can be controlled by computers.
High-performance lasers or laser diodes which are used in commercially available image-setters emit light in the wave-length ranges of between 800 to 850 nm and between 1060 and 1120 nm, respectively. Therefore, printing plate precursors, or initiator systems contained therein, which are to be imagewise exposed by means of such image-setters have to be sensitive in the near IR range. Such printing plate precursors can then basically be handled under daylight conditions which significantly facilitates their production and processing.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative printing plates, radiation-sensitive compositions are used wherein after an imagewise exposure the exposed areas are cured. In the developing step, only the unexposed areas are removed from the substrate. For positive printing plates, radiation-sensitive compositions are used whose exposed areas dissolve faster in a given developing agent than the non-exposed areas. This process is referred to as photosolubilization.
Negative-working plates typically require after imagewise exposure require a preheating step, as described for example in EP-A-0 672 544, EP-A-0 672 954 as well as U.S. Pat. No. 5,491,046 and EP-A-0 819 985. These plates require a preheating step within a very narrow temperature range which only causes a partial crosslinking of the image layer. To meet current standards regarding the number of printable copies and the resistance to press room chemicals, an additional heating step—referred to herein as a preheat step—is carried out during which the image layer is crosslinked further.
The above systems have the additional disadvantage that a relatively high laser performance (≧150 mJ/cm2) is required; for some applications, such as newsprinting, this represents a disadvantage in view of the requirement of providing a certain number of exposed printing plates within a short period of time.
U.S. Pat. No. 4,997,745 describes photosensitive compositions comprising a dye absorbing between 300 and 900 nm and a trihalomethyl-s-triazine compound. However, these compositions required a development in aqueous developers.
In U.S. Pat. No. 5,496,903 and DE-A-196 48 313, photosensitive compositions are described which in addition to a dye absorbing in the IR range comprise borate co-initiators; also, halogenated s-triazines are described as further co-initiators. Although these compositions show an improved photosensitivity, the printing plates do not meet the requirement of a long shelf-life. After only one month of storage at room temperature, the entire layer of the printing plate is cured to such a degree that an image can no longer be created after exposure and developing of the plate.
Further photopolymerizable compositions with initiator systems are described in U.S. Pat. No. 5,756,258, U.S. Pat. No. 5,545,676, JP-A-11-038633, JP-A-09-034110, U.S. Pat. No. 5,763,134 and EP-B-0 522 175.
Radiation-sensitive compositions which show both a high degree of radiation sensitivity and a sufficiently long shelf-life when used in the manufacture of printing plate precursors are presently only known in connection with UV-absorbing dyes (EP-A-0 730 201). However, printing plate precursors using such compositions have to be manufactured and processed under darkroom conditions and cannot be imagewise exposed by means of the above-mentioned lasers or laser diodes. The fact that they cannot be processed in daylight limits their possibilities of application.
U.S. Pat. No. 6,245,486 discloses radiation sensitive printing plates, including on-press developable plates. However, this patent requires compositions having an IR ablatable mask layer over a UV addressable, negative-working, on press developable, free radical polymerizable layer.
U.S. Pat. No. 6,245,481 discloses IR-ablatable, UV-photopolymerizable two-layer compositions that require IR exposure followed by UV flood irradiation.
U.S. Pat. No. 5,599,650 discloses UV addressable, negative-working, on press developable printing plates based on free radical polymerization. This patent requires a free radical quencher polymer, specifically one containing nitroxide groups, in an overcoat layer to facilitate developability.
U.S. Pat. No. 6,071,675 discloses similar printing plates to U.S. Pat. No. 5,599,650 but additionally requires adding dispersed solid particles to the imaging layer to improve on-press developability or to reduce tackiness. The solid particles include phthalocyanine pigments, which are also used as IR absorbers.
U.S. Pat. No. 6,309,792 and WO 00/48836 describe IR-sensitive compositions comprising besides a polymeric binder and a free radically polymerizable system an initiator system comprising (a) at least one compound capable of absorbing IR radiation, (b) at least one compound capable of producing radicals and (c) at least one polycarboxylic acid comprising an aromatic moiety substituted with a heteroatom selected from N, O and S and at least two carboxyl groups, wherein at least one of the carboxyl groups is bonded to the heteroatom via a methylene group. The compositions may furthermore contain a colorant for increasing the contrast of the image compared to the background after development. The compositions of WO 00/48836 require a preheat step after the exposure for sufficient hardening of the compositions. The printing plate precursors must be developed with an aqueous developer.
U.S. application Ser. No. 09/832989 describes IR sensitive compositions containing leuco dyes additional to those described in U.S. Pat. No. 6,309,792 and WO 00/48836. U.S. application Ser. No. 09/832989 requires a preheat step after IR exposure and an aqueous development step for processing.
U.S. Pat. No. 5,204,222 teaches a composition comprising polymerizable ingredients in conjunction with a polymer binder comprising a polyurethane main chain. The side chains of the polymer binder do not comprise a polyethylene oxide chain.
U.S. Pat. No. 5,800,965 teaches a composition comprising monomers of polyethylene glycol as polymerizable components. The patent does not disclose the use of polyethylene oxide chains to prepare the polymeric binders.
EP 1,117,005 discloses photopolymerizable compounds which contain polyethylene oxide chains having 1-10 ethylene oxide units. The invention is exemplified by the use of polymers having one ethylene oxide unit. With more than ten ethylene oxide units, both resolution and water resistance of cured products decrease.
There is therefore a need in the art for a printing plate and process for preparing a printing plate that does not require a preheat step or a development step, and further does not require an IR-laser ablatable layer.